Specific requirement of putrescine for the mitogenic action of juvenile hormone on adult insect neuroblasts

Abstract

Persistent neurogenesis in an adult insect brain was recently shown to be stimulated by juvenile hormone (JH). This morphogenetic hormone was also shown to act on polyamine biosynthesis. To analyze the possible involvement of polyamines in the neurogenic action of JH, two series of experiments were carried out with adult female crickets, Acheta domesticus: (i) inhibition of the first key enzyme in polyamine biosynthesis, ornithine decarboxylase, with alpha-difluoromethylornithine (alpha-DFMO), and examination of the effects of this treatment on the neuroblast proliferation response to JH; and (ii) examination of the effects of putrescine supplementation on the mitotic index of JH-deprived and alpha-DFMO-treated females. In control females, alpha-DFMO treatment, as well as JH deprivation, greatly reduced neuroblast proliferation. Putrescine supplementation in alpha-DFMO-treated insects overcame the effects of alpha-DFMO, and allowed for detection of putrescine in the neural tissue and stimulation of brain neurogenesis. In JH-deprived females, alpha-DFMO treatment completely prevented the stimulatory action of JH on neuroblast proliferation and on brain putrescine levels. By contrast, putrescine feeding of JH-deprived animals was able to mimic the stimulatory effect of JH: brain putrescine levels increased and neuroblast proliferation was restored. To our knowledge, this report demonstrates for the first time that in vivo administration of putrescine can mimic the effects of a morphogenetic hormone on adult neuroblast proliferation, and shows the importance of polyamines, especially putrescine, in the transduction of JH message in neural tissue.

Figure 1

(A) Number of mitotic cells in control females (controls), allatectomized females (CA−), α-DFMO-treated females (DFMO), and α-DFMO-fed females supplemented with putrescine (DFMO + put.). Treatments were administered from adult emergence to the day of assay. Data are the mean values ± SEM. Sample size is indicated in the columns. Columns with the same sign are not significantly different. (B) Number of mitotic cells in allatectomized females (CA−) submitted to different experimental treatments. Putrescine (put.) or α-DFMO (DFMO) were given in drinking water from emergence onwards. JH III (100 μg/10 μl paraffin oil) was injected at emergence into females, which were provided with α-DFMO (DFMO + JH) or not (JH). Data are the mean values ± SEM. Sample size is indicated in the columns. Columns with the same sign are not significantly different.

Figure 2

(A) Brain polyamine (putrescine, spermidine, and spermine) titres in control females (controls), allatectomized females (CA−), α-DFMO-treated females (DFMO), and α-DFMO-fed females supplemented with putrescine (DFMO + put.). Data are the mean values ± SEM. Sample size is indicated in the spermine columns. Columns with the same sign are not significantly different. (B) Brain polyamine (putrescine, spermidine, and spermine) titres in allatectomized females (CA−) submitted to various experimental treatments. Putrescine (put.) or α-DFMO (DFMO) were given in drinking water from emergence onwards. JH III (100 μg/10 μl paraffin oil) was injected at emergence into females, which were provided with α-DFMO (DFMO + JH) or not (JH). Data are the mean values ± SEM. Sample size is indicated in the spermine columns. Columns with the same sign are not significantly different.

Figure 3

Schematic representation of a possible mechanism of action of JH on adult neuroblast proliferation. As for vertebrate hormones, JH could act on the protooncogenes c-myc and max (–41) and induce the activation of ODC gene (42, 43). The resulting putrescine production could regulate protooncogene expression (–50) and/or act on cyclins involved in cell cycle control (51, 52).